var imgs=document.getElementById("bb").children;
var index=0;
var dingshiqi;
function aa(){
	index++;
	if(index==imgs.length){
		index=0;
	}
	for(var i=0;i<imgs.length;i++){
		imgs[i].style.cssText="z-index: 0;";
	}
	imgs[i].style.cssText="z-index: 100;"
}
dingshiqi=setInterval(aa(),1000);